The present invention relates to brine based drilling fluid system with improved fluid loss control properties at high temperatures provided by cationic polymers.
Filtration control is one of the most important properties of a drilling fluid, particularly when drilling through permeable formations where the hydrostatic pressure exceeds the formation pressure. It is important for a drilling fluid to quickly form a filter cake which effectively minimizes fluid loss, but which also is thin and dispersible enough to allow product to flow into the wellbore during production.
Filtration control additives for brines typically are nonionic water soluble polymers, such as starches, derivatized starches, gums, derivatized gums, and cellulosics. These polymers have certain advantages, but suffer from the disadvantage that they have a relatively low hydration rate in brinesxe2x80x94particularly in high density brines, where very little water actually is available to hydrate and swell the polymers.
Another disadvantage of nonionic water-soluble polymers is that they have limited temperature stability. As wells are drilled deeper, higher bottomhole tempt are encountered. Today""s drilling fluids need to maintain stable rheology and low filtration at temperatures above 300xc2x0 F. Unfortunately, the nonionic water soluble polymers currently in use are not stable at exceeding about 225xc2x0 F. with extended aging times.
Filtration control additives are needed which will quickly form a thin, dispersible filter cake, and which also have high temperature stability for prolonged period of time.
The present invention provides a drilling fluid system comprising a brine comprising a quantity of cationic copolymer comprising a ratio of acrylamide monomers to cationic derivatives of acrylamide monomers, wherein the quantity and the ratio are effective to maintain effective rheology and fluid loss control in the drilling fluid system at temperatures of at least about 250xc2x0 F. for at east about 16 hours.
The present invention provides a drilling fluid system, preferably for use as a drill-in or completion fluid, which quickly forms a thin, dispersable filter cake and which is stable for prolonged periods of time at high temperatures.
The drilling fluid system comprises an aqueous brine, preferably a xe2x80x9chigh density brinexe2x80x9d (defined below), a viscosifier, a bridging agent, a pH stabilizer, and one or more fluid loss control additive(s). A preferred fluid loss control additive comprises cationic copolymers.
The cationic copolymers of the present invention may be used as an additive in substantially any aqueous brine drilling fluid system. However, not all types of drilling fluid systems encounter extremely high temperatures. Because of this, a preferred use for the drilling fluid system of the present invention is as a drill-in or completion fluidxe2x80x94fluids which are more likely to be exposed to higher downhole temperatures for prolonged periods of time.
Preferred drill-in and completion fluids are brines having a density of at east about 9 lb/gal, most preferably xe2x80x9chigh density brines,xe2x80x9d defined herein to mean brines having a density of at least about 12-17 lb/gal. The brines may contain substantially any suitable salts, including, but not necessarily lifted to salts based on metals, such as calcium, magnesium, sodium, potassium, cesium, zinc, aluminum, and lithium. Salts of calcium and zinc are preferred. The salts may contain substantially any anions, with preferred anions being less expensive anions including, but not necessarily limited to chlorides, bromides, formates, acetates, and nitrates. Most preferred salts are calcium bromide and zinc bromide.
For purposes of the present invention, the term xe2x80x9ccationic copolymersxe2x80x9d is defined to refer to cationic copolymers which provide effective rheology and filtration control at temperatures greater than about 250xc2x0 F., preferably about 300xc2x0 F., most preferably about 325xc2x0 F., for about 16 hours, preferably for about 48 hours or more. For purposes of the present application, effective rheology is defined to mean structure which is sufficient to suspend bridging agents but not excessive so as to cause high equivalent circulating densities. Effective filtration control is defined to mean control which provides a low filtration rate with a thin, dispersable filter cake.
Preferred cationc copolymr include, but are not necessarily limited to copolymers comprising, and preferably consisting essentially of monomers of acrylamide and monomers of a cationic derivative of acrylamide. Preferred cationic derivatives of acrylamide for use in such copolymers are quaternary salts of N,N-dialkylaminoacrylamide wherein the size of the alkyl groups is limited by solubility to about 5, preferably about 4, most preferably about 1-3 carbon atoms. A preferred cationic quaternary salt is quaternary methyl N,N-dimethylaminoethylmethacrylamide. The copolymers preferably comprise a ratio of from about 3:1 to about 1:1 of the cationic monomer.
Cationic copolymers suitable for use in the present invention are commercially available from Fritz Industries, Inc., Dallas, Tex., under the name EXP-8 EMULSION POLYMER. In order to achieve the desired rheological and filtration control, the fluid should contain from about 1 lb/bbl to about 10 lb/bbl of a 35% active solution of the cationic copolymer in a suitable carrier, such as oil, which translates to about 0.35 to about 3.5 lb/bbl active cationic copolymer.
The cationic copolymers can be used alone or used in conjunction with a different type of fluid loss additive, preferably a 2-amino-2-methyl propane sulfonic acid (AMPS) additive, such as KEM SEAL PLUSH(copyright), available from Baker Hughes INTEQ. Where a combination of cationic copolymer and another fluid loss additive is used, the ratio of cationic copolymer to the other fluid loss additive preferably is about 2:1 to about 1:2, most preferably about 1:1.
The system preferably includes bridging agents to bridge the pores in the formation. Suitable bridging agents include, but are not necessarily limited to ground marble or calcium carbonate particles, such as MIL-CARB(copyright), available from Baker Hughes INTEQ. Preferred calcium carbonate particles have a mean particle size of about 30 microns. Calcium carbonate has the advantage that it is acid soluble, and therefore can be removed from the formation by acid flushing. If calcium carbonate is used as the bridging agent, about 50 pounds should be used per barrel of brine.
The system also preferably includes a viscosifier, such as SALT WATER GEL(copyright), available from Baker-Hughes INTEQ, Houston, Tex. A preferred viscosifier is EXP-77, a cellulosic blend, also available from Baker Hughes INTEQ.
Finally, the system includes a suitable material for adjusting the pH of the system to from about 9 to about 10. Suitable materials include, but are not necessarily limited to hydrous oxides of divalent cations. A preferred material is MgO.
A preferred basic formulation for a drilling fluid system according to the present invention is given in the following table:
The invention will be more clearly understood with reference to the following examples, which are illustrative only and should not be construed as limiting the present invention.